This invention relates to compositions for paints and, printing inks having a good coating property and particularly, to compositions for paints and printing inks containing a vinyl-substituted aromatic hydrocarbon/conjugated diene block copolymer as a binder, having good wetting to inorganic pigments and good dispersibility therefor.
Styrene-butadiene type resins have, hitherto, been used as a binder for paints for concrete, vessels and other surfaces and for printing inks, in which such properties as water resistance, chemical resistance and abrasion resistance are required.
Since these styrene-butadiene type resins are usually produced by a radical polymerization method, they have a wide distribution of molecular weight, including a considerable amount of low-molecular weight polymers and, further, have a polymer chain configuration having styrene and butadiene units arranged at random therein. Therefore, a coating or film formed by these resins is markedly deteriorated in mechanical strength and, also, lowered in such properties as water resistance, weather resistance and chemical resistance.
To improve the styrene-butadiene resins film strength, various methods were proposed, but such have provided products unsuitable for use as a binder for paints and printing inks. By way of example, a high impact polystyrene with improved film strength is obtained by grafting polystyrene to elastomeric polymers. However, when this resin is dissolved in a solvent such as xylene and formed to a casting film, elastomer particles deposit out on the film face and, accordingly, the film obtained has an extremely bad appearance. Similarly, with ABS resins obtained by copolymerization with acrylic monomers, when these resins are dissolved in a solvent such as xylene and formed to a casting film, gelled elastomer particles deposit out on the film face and thus the film obtained is markedly deteriorated in the the appearance.
On the other hand, it is known that block copolymers having a regulated molecular configuration are obtained by copolymerizing vinyl-substituted hydrocarbon and conjugated diene monomers by an anionic living polymerization method using an alkali metal or organoalkali metal initiator. It is also known that block copolymers with various properties are obtained by varying the molecular weight, molecular weight distribution and the composition ratio of monomers. For example, with a styrene/butadiene ratio ranging from about 25/75 to 50/50, the resulting block copolymer is a transparent, thermoplastic elastomer, which is used for shoe soles, hot melt adhesives, plastic blends and the like.
Also, block copolymers having a styrene/butadiene ratio of from about 50/50 to 70/30 are a transparent, soft resin and can be used for stretchable films, toy materials and the like.
Further, block copolymers having a styrene/butadiene ratio of from about 75/25 to 90/10 are a rigid, transparent resin, which are improved in mechanical strength such as impact resistance and in properties such as cold resistance.
We have studied styrene-butadiene type resins prepared by anionic living polymerization as a binder for paints and printing inks and, as a result, we have found that coating compositions containing these resins as the binder give a coat or film having a smooth surface without the occurrence of gelling and that these coating compositions are thixotropic, have good coating workability and are anticorrosive. Also, we have found that a printing ink prepared using these resins as a binder is useful for inks exhibiting a delustering, delicate colour tone.
However, block copolymers obtained by anionic living polymerization exhibit poor wetting to inorganic pigments and, accordingly, the dispersibility of pigments in the paint and printing ink compositions is deteriorated. Therefore, in the case of using compositions having a practical concentration of pigments, the resulting film or coat is considerably deteriorated in properties such as gloss and water resistance as compared with the case of using compositions prepared using copolymer resins obtained by radical polymerization.
Thus, styrene/butadiene copolymers obtained by living polymerization are highly superior in the mechanical and chemical strength of the resin itself to those obtained by radical polymerization, but are nevertheless limited to a particular field of use for paint and printing ink composition.
U.S. Pat. Nos. 3,108,994 and 3,135,716 disclose a method for the production of a terminally reactive polymer by reacting living polymers with a suitable reactant, in which mercapto, hydroxy or acidic groups are introduced as a reactive group, and the resulting terminally reactive polymer is used for cross linking.
Also, dispersions of titanium dioxide particles in toluene stabilized by partially carboxylated styrene-butadiene block copolymer are reported in Advan. Chem. Ser., Vol. 99, PP 379-396 (1971), "Dispersion of Solid Particles in Organic Media"by G. E. Molan and E. H. Richardson. The carboxylation is effected by adding thioglycolic acid to the main chain of the polymer.